I'd say it's a decent first attempt. Here are my two cents:
a) It lacks proper shading.
b) the stance of the feet is odd, not like you would see in real life and is a bit unnatural.
c) proportions or way off
d) the way his sword is being held is strange
e) the helmet is lopsided
f) the hair is a bit stringy and lacks structure at all
g) designs and armor as a whole looks flat
h) the cape looks odd and not really like a cape
i) his stance overall is a bit too straight
I hope I helped you. In no way was I trying to be mean or offend you, I just noticed several things about your artwork that could be fixed. I think if you fixed it up a bit, it could be a nice drawing. Look around on the internet for other pictures/sketches of Arthas to get a feel of what he looks like and the way he stands and acts.